1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly to an optical disk apparatus capable of discriminating a loaded optical disk from a plurality of optical disks having different thicknesses and selecting a pickup lens most suitable for the discriminated optical disk.
2. Description of the Related Art
A conventional optical disk apparatus for data recording, reproducing and erasing has: an optical focussing system in which a laser beam is focussed via a pickup lens to a loaded optical disk, and light reflected from the optical disk is detected with a photodetector; and a focus pull-in control system having an actuator for converging the laser beam to the optical disk.
FIG. 7 is a block diagram showing the structure of such a conventional optical disk apparatus. This optical disk apparatus has: a photodetector 31 having a total of four unit elements A, B, C and D; amplifiers 32, 33, 34 and 35 for amplifying respective signals detected with the unit elements A, B, C and D; an RF amplifier 36 for amplifying a sum of all the outputs of the amplifiers 32, 33, 34 and 35; a focus error (hereinafter abbreviated as FE) amplifier 37 for amplifying a sum of the outputs of the amplifiers 33 and 35 subtracted by a sum of the outputs of the amplifiers 32 and 34; a signal processing circuit 50 for processing an RF signal 38 output from the RF amplifier 36 to obtain a necessary signal such as an audio signal; a D/A converter 51 for converting a digital signal output from the signal processing circuit 50 into an analog signal; an amplifier 52 for amplifying an output of the D/A converter 51, and a speaker 53 for receiving an output of the amplifier 52 and producing sounds. The optical disk apparatus further includes: an RF comparator 40 for receiving an RF signal 38 generally called a sum signal as its non-inverting input (+) and a voltage divided by a variable resistor 42 as its inverting input (-) and outputting a focus-ok (FOK) signal 55; an FZC comparator 41 for receiving an FE signal 39 from the FE amplifier 37 as its non-inverting input (+) and a voltage divided by resistors 43 and 44 as its inverting input (-) and outputting a focus zero cross (FZC) signal 56; a servo ON/OFF switch 46, a resistor 45 connected between one end of the switch 46 and an output of the FE amplifier 37; a servo circuit 47 for receiving a signal at the one end of the switch 46 and a signal from a controller 54; a driver 48 for power amplifying a signal output from the servo circuit 47; an actuator 49 connected to an output of the driver 48 and made of a focus coil; and the controller 54 made of a microcomputer for receiving the FOK signal 55 and the FZC signal 56 and controlling the servo system including the switch 46, servo circuit 47, and the like. A combination of the FOK signal and FZC signal is called a focus signal.
The variable resistor 42 is connected between a power source voltage +V and the ground, and is used for controlling an FOK level so that the focus point of a reflection film of the optical disk is detected without detecting the surface of the optical disk. The resistors 43 and 44 are serially connected between the power source voltage +V and the ground, and is used for presetting the bias of the inverting input (-) to a predetermined voltage value so that the zero cross point of the FE signal 39 is correctly detected. The block diagram of FIG. 7 described above is mainly pertains to a focus pull-in control system.
The operation of the optical disk apparatus shown in FIG. 7 will be described with reference to the flow chart of FIG. 8 illustrating the control procedure for the optical disk and with reference to the timing chart of FIG. 9 showing the waveforms of signals at various circuit portions. First, at Step S1 a laser power source is turned on and a laser beam is applied to a loaded optical disk. Next, at Step S2 the focus servo system starts moving up a pickup lens toward the optical disk at a constant speed. Therefore, the in-focus position gradually moves toward the inner region of the optical disk.
In this case, a small peak appears on the RF signal 38 at a position P1 and at a timing t1 when the in-focus position reaches the surface of the optical disk. This small peak waveform has a level of the FOK level or lower set to the RF comparator 40 so that the FOK signal 55 does not change. An S-character or inverted S-character curve appears on the FE signal 39 so that the FZC signal 56 of a negative pulse having a falling edge at the focus zero cross point is output. The two signals, FOK and FZC signals 55 and 56, are collectively called a focus signal as described earlier. This position P1 corresponds to the in-focus position of the optical disk surface. Since it is not meaningless to stop the servo system and fix the focus at this timing, the switch 46 maintains an OFF state so that the servo system continues its operation without advancing to Step S5.
A position P2 at a timing t2 when the FOK signal 55 is detected at Step S3 corresponds to the in-focus position at the reflection layer in the inner region of the optical disk. In this case, since the RF signal 38 has a level higher than the FOK level, an FOK signal 55 is output which is a positive pulse having a width corresponding to the period while the RF signal 38 takes a level higher than the FOK level. In this case, the FE signal 39 has a large S-character impulse which is shaped by the FZC comparator 41 and output as the FZC signal 56 having a falling edge at the zero cross point.
Since the FZC signal 56 takes an L (low) level at Step S4 and the FOK signal 55 takes an H (high) level at Step S3, the flow advances to Step S5 whereat the focus servo loop is closed so that motion of the pickup lens is stopped and the position P2 is established as the focus-on-point which is the in-focus point at the reflection layer.
As the lens is moved at a constant speed, there is a point at which the in-focus is obtained instantaneously. This in-focus point corresponds to the zero cross point while the RF signal 38 is higher than the FOK level and while the amplitude of the focus error signal is lower than a threshold value E. It is known that if the objective lens is positioned in the area corresponding to this period, the focus pull-in can be performed always stably.
In the structure described above, the FOK level is set by the RF comparator 40. There is a case wherein there is only a small difference between the peak values of the RF signals 38 reflected from the surface of an optical disk and from the reflection layer in the inner region of the optical disk. In such a case, the pulse of the FOK signal 55 is generated for both the peak signals of the RF signals. Therefore, the disk surface may be erroneously judged to be the reflection layer surface. Alternatively, the pulse of the FOK signal 55 may not generated for both the peak signals so that the unload state of an optical disk may be erroneously judged. As above, inability of setting a sufficient voltage margin of the FOK level may result in focus pull-in control with less reliability.
Also in the above structure, the focus control is performed by a pickup lens used in common for optical disks of one kind having generally the same focus-on-point. If an optical disk of another kind having a thickness greatly different from the one kind is loaded, it is impossible to use the same lens for the focus control in excess of a refraction limit because of the spherical aberration.
FIGS. 10A to 10C are cross sectional views of optical disks of three kinds. The optical disk shown in FIG. 10A is called a CD (compact disk) having a thickness of about 1.2 mm and an aluminum reflection film 60 formed on the main surface of a polycarbonate substrate 61. An optical disk shown in FIG. 10B is called a DVD (digital versatile disk) having a thickness of about 0.6 mm and an aluminum reflection film 64 formed on the main surface of a second polycarbonate substrate 63 and a first substrate 62 stacked on the surface of the reflection film 64. The optical disk shown in FIG. 10C is called a two-layer DVD having a thickness of about 0.6 mm and a reflection film 67, an intermediate layer (transparent material) and a semi-transparent film 68 interposed between first and second substrates 65 and 66. It is desired to use the same optical disk apparatus for the reproduction and recording of optical disks of three different kinds.
A conventional optical disk apparatus having a function of discriminating between optical disks of a plurality of different kinds is disclosed in Japanese Patent Laid-open HEI 5-54406. This optical disk can discriminate between different kinds of optical disks without using a specific detector, by measuring a disk substrate thickness with measuring means which measures the time interval of two S-character waves on the focus error signal while focus position control means moves the objective lens near to the disk surface.
With this structure, however, the time interval depends on the motion speed of the objective lens. Therefore, if the motion speed fluctuates, the time interval changes even if the disk thickness is the same so that correct measurement of the disk thickness is difficult.